Pressurized water reactor with upper vessel section providing both pressure and flow control

ABSTRACT

A pressurized water reactor (PWR) includes a vertical cylindrical pressure vessel having a lower portion containing a nuclear reactor core and a vessel head defining an integral pressurizer. A reactor coolant pump (RCP) mounted on the vessel head includes an impeller inside the pressure vessel, a pump motor outside the pressure vessel, and a vertical drive shaft connecting the motor and impeller. The drive shaft does not pass through the integral pressurizer. The drive shaft passes through a vessel penetration of the pressure vessel that is at least large enough for the impeller to pass through.

BACKGROUND

The following relates to the nuclear reactor arts, electrical powergeneration arts, nuclear reactor control arts, nuclear electrical powergeneration control arts, thermal management arts, and related arts.

In nuclear reactor designs for steam generation, such as boiling waterreactor (BWR) and pressurized water reactor (PWR) designs, a radioactivereactor core is immersed in primary coolant water at or near the bottomof a pressure vessel. In BWR designs heat generated by the reactor coreboils the primary coolant water creating steam that is extracted bycomponents (e.g., steam separators, steam dryer, or so forth) located ator near the top of the pressure vessel. In PWR designs the primarycoolant is maintained in a compressed or subcooled liquid phase and iseither flowed out of the pressure vessel into an external steamgenerator, or a steam generator is located within the pressure vessel(sometimes called an “integral PWR” design). In either design, heatedprimary coolant water heats secondary coolant water in the steamgenerator to generate steam. An advantage of the PWR design is that thesteam comprises secondary coolant water that is not exposed to theradioactive reactor core.

In either a BWR design or a PWR design, the primary coolant flowsthrough a closed circulation path. Primary coolant water flowing upwardthrough the reactor core is heated and rises through a central region tothe top of the reactor, where it reverses direction and flows downwardback to the reactor core through a downcomer annulus defined between thepressure vessel and a concentric riser structure. This is a naturalconvection flow circuit for such a reactor configuration. However, forhigher power reactors it is advantageous or necessary to supplement orsupplant the natural convection with motive force provided byelectromechanical reactor coolant pumps.

In a conventional approach, glandless pumps are used, in which a unitarydrive shaft/impeller subassembly is rotated by a pump motor. This designhas the advantage of not including any seals at the drive shaft/impellerconnection (hence the name “glandless”). For nuclear reactors, a commonimplementation is to provide a unitary reactor coolant pump comprisingthe sealless drive shaft/impeller subassembly, the motor (including thestator, a rotor magnet or windings, and suitable bearings or other driveshaft couplings), and a supporting flange that supports the motor andincludes a graphalloy seal through which the drive shaft passes toconnect the pump motor with the impeller. The reactor coolant pump isinstalled by inserting the impeller through an opening in the reactorpressure vessel and securing the flange over the opening. Wheninstalled, the impeller is located inside the pressure vessel and thepump motor is located outside of the pressure vessel (and preferablyoutside of any insulating material disposed around the pressure vessel).Although the motor is outside of the pressure vessel, sufficient heatstill transfers to the pump motor so that dedicated motor cooling istypically provided in the form of a heat exchanger or the like. Externalplacement of the pump motor simplifies electrical power connection andenables the pump motor to be designed for a rated temperaturesubstantially lower than that of the primary coolant water inside thepressure vessel. Only the impeller and the impeller end of the driveshaft penetrate inside the pressure vessel.

Disclosed herein are improvements that provide various benefits thatwill become apparent to the skilled artisan upon reading the following.

BRIEF SUMMARY

In one aspect of the disclosure, an apparatus comprises a pressurizedwater reactor (PWR) including: a cylindrical pressure vessel with itscylinder axis oriented vertically; a nuclear reactor core disposed inthe cylindrical pressure vessel; a separator plate disposed in thecylindrical pressure vessel that separates the pressure vessel to definean integral pressurizer containing a pressurizer volume disposed abovethe separator plate and a reactor vessel portion defining a reactorvolume disposed below the separator plate and containing the nuclearreactor core, wherein the separator plate restricts but does notcompletely cut off fluid communication between the pressurizer volumeand the reactor volume; and a reactor coolant pump including (i) animpeller disposed inside the pressure vessel in the reactor volume, (ii)a pump motor disposed outside of the pressure vessel, and (iii) a driveshaft operatively connecting the pump motor with the impeller, wherein(1) at least a portion of the pump motor is disposed above the separatorplate, (2) no portion of the reactor coolant pump is disposed in thepressurizer volume, and (3) the drive shaft passes through an opening inthe pressure vessel that is at least large enough to pass the impeller.

In another aspect of the disclosure, a method comprises installing areactor coolant pump comprising a pump motor, a driveshaft, an impeller,and a mounting flange on a pressurized water reactor (PWR) comprising apressure vessel and a nuclear reactor core disposed in the pressurevessel, the installing including: pre-assembling the pump motor, thedriveshaft, the impeller, and the mounting flange outside of thepressure vessel to form a pump assembly as a unit disposed outside ofthe pressure vessel in which the pump motor is connected with theimpeller by the driveshaft; inserting the impeller and the driveshaft ofthe pump assembly through an opening of the pressure vessel while thepump motor remains outside of the pressure vessel; and securing theflange of the pump assembly to an outside of the pressure vessel tomount the pump assembly on the pressure vessel; wherein the insertingand securing mounts the pump assembly on the pressure vessel with thedrive shaft of the pump assembly oriented vertically.

In another aspect of the disclosure, the reactor coolant pump of theimmediately preceding paragraph further comprises a pump diffuser thatis not a component of the unitary pump assembly formed by thepre-assembling, and the installing of the immediately precedingparagraph further comprises disposing the pump diffuser inside thepressure vessel in an operation other than the inserting and thesecuring operations.

In another aspect of the disclosure, an apparatus comprises a reactorcoolant pump including a pump assembly and a pump diffuser. The pumpassembly includes a pump motor, an impeller, and a driveshaft thatoperatively connects the pump motor and the impeller as said pumpassembly. The pump diffuser is configured to receive the impeller. Thepump diffuser is not secured with the pump assembly.

In another aspect of the disclosure, an apparatus comprises a reactorcoolant pump as set forth in the immediately preceding paragraph, and apressurized water reactor (PWR) including a cylindrical pressure vesselwith its cylinder axis oriented vertically, a nuclear reactor coredisposed in the cylindrical pressure vessel, and a separator platedisposed in the cylindrical pressure vessel that separates the pressurevessel to define an integral pressurizer volume disposed above theseparator plate and a reactor vessel portion containing the nuclearreactor core disposed below the separator plate, The reactor coolantpump is mounted on the cylindrical pressure vessel of the PWR with theimpeller disposed in the pump diffuser and with at least a portion ofthe pump motor being disposed above the separator plate. No portion ofthe reactor coolant pump passes through the integral pressurizer volume.

In another aspect of the disclosure, an apparatus comprises a reactorcoolant pump and a pressurized water reactor (PWR). The reactor coolantpump includes a pump motor, an impeller, a driveshaft that operativelyconnects the pump motor and the impeller, and a pump diffuser. The pumpmotor, the impeller, the driveshaft, and the pump diffuser are securedtogether as a unitary pump assembly with the impeller disposed in thepump diffuser. The PWR includes a cylindrical pressure vessel with itscylinder axis oriented vertically, a nuclear reactor core disposed inthe cylindrical pressure vessel, and a separator plate disposed in thecylindrical pressure vessel that separates the pressure vessel to definean integral pressurizer volume disposed above the separator plate and areactor vessel portion containing the nuclear reactor core disposedbelow the separator plate. The unitary pump assembly is mounted on thecylindrical pressure vessel of the PWR with at least a portion of thepump motor disposed above the separator plate and with no portion of thereactor coolant pump passing through the integral pressurizer volume.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for purposes of illustratingpreferred embodiments and are not to be construed as limiting theinvention.

FIG. 1 diagrammatically shows a side sectional view of a pressurizedwater reactor (PWR) including an integral pressurizer and reactorcoolant pumps (RCPs).

FIG. 2 diagrammatically shows a perspective view of the upper vesselsection of the PWR of FIG. 1.

FIG. 3 diagrammatically shows a side sectional view of the vessel headregion of the PWR of FIG. 1 including the integral pressurizer and RCPs.

FIG. 4 diagrammatically shows a side sectional view of one of thereactor coolant pumps (RCPs).

FIG. 5 diagrammatically shows a perspective view of the upper vesselsection of the PWR of FIG. 1 with the closure opened to remove thevessel head from the remainder of the vessel, and with the vessel headand remainder of the vessel both tilted to reveal selected internalcomponents.

FIGS. 6 and 7 diagrammatically shows a perspective view and enlargedsectional perspective view, respectively, of an alternative embodimentof the upper vessel section that omits the closure for removing thevessel head and that employs a differently shaped integral pressurizer.

FIG. 8 diagrammatically shows a perspective view of an alternativeembodiment of the vessel head region of the PWR of FIG. 1 that includesanother alternative RCP embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1-4, a pressurized water reactor (PWR) includesa cylindrical pressure vessel 10. As used herein, the phrase“cylindrical pressure vessel” indicates that the pressure vessel has agenerally cylindrical shape, but may in some embodiments deviate from amathematically perfect cylinder. For example, the illustrativecylindrical pressure vessel 10 has a circular cross-section of varyingdiameter along the length of the cylinder, and has rounded ends, andincludes various vessel penetrations, vessel section flange connections,and so forth. The cylindrical pressure vessel 10 is mounted in anupright position having an upper end 12 and a lower end 14. However, itis contemplated for the upright position to deviate from exact verticalorientation of the cylinder axis. For example, if the PWR is disposed ina maritime vessel then it may be upright but with some tilt, which mayvary with time, due to movement of the maritime vessel on or beneath thewater. The PWR further includes a diagrammatically indicated radioactivenuclear reactor core 16 comprising a mass of fissile material, such as amaterial containing uranium oxide (UO₂) that is enriched in the fissile²³⁵U isotope, arranged fuel rod bundles or so forth disposed in a fuelbasket or other support assembly configured to mount in suitablemounting brackets or retention structures of the pressure vessel 10(core mounting features not shown). Reactivity control is provided by adiagrammatically indicated control rod system 18, typically comprisesassemblies of control rods that are mounted on connecting rods, spiders,or other support elements. The control rods comprise a neutron absorbingmaterial and the control rod assemblies (CRA's) are operativelyconnected with control rod drive mechanism (CRDM) units thatcontrollably insert or withdraw the control rods into or out of thereactor core 16 to control or stop the chain reaction. As with thereactor core 16, the control rod system 18 is shown diagrammatically andindividual components such as individual control rods, connecting rods,spiders, and CRDM units are not shown. The diagrammatically illustratedcontrol rod system is an internal system in which the CRDM units aredisposed inside the pressure vessel 10. Some illustrative examples ofinternal control rod system designs include: Stambaugh et al., “ControlRod Drive Mechanism for Nuclear Reactor”, U.S. Pub. No. 2010/0316177 A1published Dec. 16, 2010 which is incorporated herein by reference in itsentirety; and Stambaugh et al., “Control Rod Drive Mechanism for NuclearReactor”, Intl Pub. WO 2010/144563 A1 published Dec. 16, 2010 which isincorporated herein by reference in its entirety. Alternatively,external CRDM units may be used—however, external CRDM units requiremechanical penetrations through the top or bottom of the pressure vessel10 to connect with the control rods.

In its operating state, the pressure vessel 10 of the PWR containsprimary coolant water that serves as primary coolant and as a moderatormaterial that thermalizes neutrons. The illustrative PWR includes anintegral pressurizer as follows. A separator plate 20 is disposed in thecylindrical pressure vessel 10. The separator plate 20 separates thepressure vessel 10 to define: (1) an integral pressurizer 22 containinga pressurizer volume disposed above the separator plate 20; and (2) areactor vessel portion 24 defining a reactor volume disposed below theseparator plate 20. The nuclear reactor core 16 and the control rodsystem 18 is disposed in the reactor volume. The separator plate 20restricts but does not completely cut off fluid communication betweenthe pressurizer volume and the reactor volume. As a result, pressure inthe pressurizer volume communicates to the reactor volume, so that theoperating pressure of the reactor volume can be adjusted by adjustingpressure in the pressurizer volume. Toward this end, a steam bubble ismaintained in the upper portion of the pressurizer volume, and theintegral pressurizer 22 includes heater elements 26 for applying heat toincrease the temperature (and hence increase pressure) in the integralpressurizer 22. Although not shown, spargers may also be provided toinject cooler steam or water to lower the temperature (and hencepressure) in the integral pressurizer 22. In a PWR the primary coolantwater is maintained in a subcooled state. By way of illustrativeexample, in some contemplated embodiments the primary coolant pressurein the sealed volume of the pressure vessel 10 is at a pressure of about2000 psia and at a temperature of about 300-320° C. Again, this ismerely an illustrative example, and a diverse range of other subcooledPWR operating pressures and temperatures are also contemplated.

The reactor core 16 is disposed in the reactor volume, typically nearthe lower end 14 of the pressure vessel 10, and is immersed in theprimary coolant water which fills the pressure vessel 10 except for thesteam bubble of the integral pressurizer 22. (The steam bubble alsocomprises primary coolant, but in a steam phase). The primary coolantwater is heated by the radioactive chain reaction occurring in thenuclear reactor core 16. A primary coolant flow circuit is defined by acylindrical central riser 30 disposed concentrically with and inside thecylindrical pressure vessel 10, and more particularly in the reactorvolume. Heated primary coolant water rises upward through the centralriser 30 until it reaches the top of the riser, at which point itreverses flow and falls through a downcomer annulus 32 defined betweenthe cylindrical central riser 30 and the cylindrical pressure vessel 10.At the bottom of the downcomer annulus 32 the primary coolant water flowagain reverses and flows back upward through the nuclear reactor core 16to complete the circuit.

In some embodiments, an annular internal steam generator 36 is disposedin the downcomer annulus 32. Secondary coolant water flows into afeedwater inlet 40 (optionally after buffering in a feedwater plenum),through the internal steam generator 36 where it is heated by proximateprimary coolant in the downcomer annulus 32 and converted to steam, andthe steam flows out a steam outlet 42 (again, optionally after bufferingin a steam plenum). The output steam may be used for driving a turbineto generate electricity or for some other use (external plant featuresnot shown). A PWR with an internal steam generator is sometimes referredto as an integral PWR, an illustrative example of which is shown inThome et al., “Integral Helical Coil Pressurized Water Nuclear Reactor”,U.S. Pub. No. 2010/0316181 A1 published Dec. 16, 2010 which isincorporated herein by reference in its entirety. While this publicationdiscloses a steam generator employing helical steam generator tubes,other tube geometries including straight (e.g., vertical) once-throughsteam generator tubes, or recirculating steam generators, or U-Tubesteam generators, or so forth are also contemplated.

In embodiments disclosed herein, circulation of the primary coolantwater is assisted or driven by reactor coolant pumps (RCPs) 50. Withparticular reference to FIG. 4, each reactor coolant pump (RCP) 50includes: an impeller 52 disposed inside the pressure vessel 10 (andmore particularly in the reactor volume); a pump motor 54 disposedoutside of the pressure vessel 10; and a drive shaft 56 operativelyconnecting the pump motor 54 with the impeller 52. At least a portion ofthe pump motor 54 is disposed above the separator plate 20, and noportion of the reactor coolant pump 50 is disposed in the pressurizervolume of the integral pressurizer 22. Each RCP 50 of the embodiment ofFIGS. 1-4 further includes an annular pump casing or diffuser 58containing the impeller 52.

Locating the RCPs 50 proximate to the integral pressurizer 22 places theopenings in the pressure vessel 10 for passage of the drive shafts 56 atelevated positions. This elevated placement reduces the likelihood ofsubstantial primary coolant loss in the event of a loss of coolantaccident (LOCA) involving the RCPs 50. Moreover, the impellers 52operate at the “turnaround” point of the primary coolant flow circuit,that is, at the point where the primary coolant water reverses flowdirection from the upward flow through the central riser 30 to thedownward flow through the downcomer annulus 32. Since this flow reversalalready introduces some flow turbulence, any additional turbulenceintroduced by operation of the RCPs 50 is likely to be negligible. TheRCPs 50 also do not impede natural circulation, which facilitates theimplementation of various passive emergency cooling systems that relyupon natural circulation in the event of a loss of electrical power fordriving the RCPs 50. Still further, the RCPs 50 are also far away fromthe reactor core 16 and hence are unlikely to introduce flow turbulencein the core 16 (with its potential for consequent temperaturevariability).

On the other hand, the placement of the RCPs 50 at the elevated positionhas the potential to introduce turbulence in the primary coolant waterflow into the internal steam generator 36. To reduce any such effect, inthe embodiment of FIGS. 1-4 the RCPs 50 are buffered by an inlet plenum60 and an outlet plenum 62. Primary coolant water flowing out of the topof the cylindrical central riser 30 flows into the inlet plenum 60 wherethe flow reverses direction, aided by the RCPs 50 which impel theprimary coolant water to flow downward into the downcomer annulus 32.Said another way, the RCPs 50 discharge primary coolant into the outletplenum 62 which separates the RCPs 50 from the internal steam generator36. Optionally, a flow diverter element or structure may be provided ator proximate to the top of the central riser 30 to assist in the flowreversal. In the illustrative embodiment, a flow diverter screen 63serves this purpose; however, in other embodiments other diverterelements or structures may be used. By way of additional illustrativeexamples, the flow diverter or structure may be embodied by sideopenings near the top of the central riser, or by shaping the separatorplate to serve as a flow diverter. Alternatively, a flow diverterstructure may be on the outlet plenum 62.

The RCPs 50 output impelled primary coolant water into the output plenum62 which buffers flow from the pumps into the annular steam generator36. The primary coolant flows from the outlet plenum 62 either into thesteam generator tubes (in embodiments in which the higher pressureprimary coolant flows inside the steam generator tubes) or into a volumesurrounding the steam generator tubes (in embodiments in which thehigher pressure primary coolant flows outside the steam generatortubes). In either case, the primary coolant flow from the RCPs 50 intothe steam generator 36 is buffered so as to reduce flow inhomogeneity.Additionally, because each RCP 50 outputs into the outlet plenum 62 andis not mechanically connected with an inlet of the internal steamgenerator 36, the failure of one RCP 50 is less problematic. (Bycomparison, if the RCPs are mechanically coupled into specific inlets ofthe steam generator, for example by constructing the pump casing so thatits outlet is coupled with an inlet of the steam generator, then thefailure of one RCP completely removes the coupled portion of the steamgenerator from use).

The illustrative RCPs 50 of FIGS. 1-5 are mounted using relatively smallopenings in the pressure vessel 10. In particular, in some embodimentsthe opening through which the driveshaft 56 passes is too small for thepump casing 58 to pass through.

With reference to FIG. 5, to enable the disclosed approach in which theopening is too small for the pump casing 58 to pass through, thepressure vessel 10 includes a closure at or below the separator plate 20and (in the illustrative example) at or above the top of the internalsteam generator 36. The closure includes mating flanges 64A, 64B thatseal together in mating fashion with suitable fasteners such as acombination of cooperating tension nuts and tension studs. In this way,a head 10H of the pressure vessel 10 can be removed from the remainderof the pressure vessel 10 by opening the closure 64 (e.g., by removingthe fasteners) and lifting off the vessel head via lifting lugs 69 thusseparating the flanges 64A, 64B (see FIG. 5, but note that FIG. 5diagrammatically shows the head 10H and vessel remainder each tilted toreveal internal components; whereas, typically the head 10H is removedby lifting it straight up, i.e. vertically, using a crane or the likeand then optionally moving the lifted head 10H laterally to a dockinglocation). The vessel head 10H defines the integral pressurizer 22 andalso includes the portion of the pressure vessel 10 that supports theRCPs 50. Therefore, removing the head 10H of the pressure vessel 10simultaneously removes the integral pressurizer 22 and the RCPs 50.Removal of the vessel head 10H exposes the upper and lower surfaces ofthe outlet plenum 62 and provides access from below to the pump casings58. Thus, during a maintenance period during which the pressure vessel10 is depressurized and the vessel head 10H removed, the pump casings 58could be installed or replaced if needed.

The RCP 50 can be installed as follows. The pump casings 58 aretypically installed first. This can be done during manufacture of thevessel head 10H or at any time prior to installation of the vessel head10H to form the complete pressure vessel 10. The pump motor 54, impeller52, and connecting drive shaft 56 are pre-assembled as a unit, and insome embodiments may be a commercially available pump such as aglandless pump of the type used in boil water reactor (BWR) systems. Thepump assembly 52, 54, 56 is mounted as a unit at an opening of thepressure vessel 10 by inserting the impeller 52 and drive shaft 56 intothe opening and bolting a mounting flange 70 of the pump assembly to thepressure vessel 10 with the mounted pump motor 54 located outside of thepressure vessel 10 and supported on the pressure vessel 10 by themounting flange 70. In an alternative assembly sequence, it iscontemplated to mount the pump assembly 52, 54, 56 as a unit onto thepressure vessel 10 prior to installation of the vessel head 10H to formthe complete pressure vessel 10 and either before or after installationof the pump casings 58.

By employing the illustrative embodiment in which the opening for theRCP 50 is too small for the pump casing 58 to pass, these openings aremade small so as to minimize the likelihood and extent of a loss ofcoolant accident (LOCA) at these openings. However, the approach stillenables use of a commercially available unitary pump assembly includingthe pump motor 54, impeller 52, and drive shaft 56 secured together assaid unitary pump assembly. The pump casing 58, on the other hand, isnot secured with the unitary pump assembly 52, 54, 56.

The number of RCPs 50 is selected to provide sufficient motive force formaintaining the desired primary coolant flow through the primary coolantcircuit. Additional RCPs 50 may be provided to ensure redundancy in theevent of failure of one or two RCPs. If there are N reactor coolantpumps (where N is an integer greater than or equal to 2, for exampleN=12 in some embodiments) then they are preferably spaced apart evenly,e.g. at 360°/N intervals around the cylinder axis of the cylindricalpressure vessel 10 (e.g., intervals of 30° for N=12). The externallymounted pump motors 54 are advantageously spaced apart from the hightemperature environment inside the pressure vessel 10. Nonetheless,substantial heat is still expected to flow into the pump motors 54 byconduction through the flanges 70 and by radiation/convection from theexterior of the pressure vessel 10. Accordingly, in the illustrativeembodiment the RCPs 50 further include heat exchangers 74 for removingheat from the pump motors 54. Alternative thermal control mechanisms canbe provided, such as an open-loop coolant flow circuit carrying water,air, or another coolant fluid. Moreover, it is contemplated to omit suchthermal control mechanisms entirely if the pump motors 54 are rated forsufficiently high temperature operation.

Another advantage of the illustrative configuration is that the pumpmotor 54 of the RCP 50 is mounted vertically, with the drive shaft 56vertically oriented and parallel with the cylinder axis of thecylindrical pressure vessel 10. This vertical arrangement eliminatessideways forces on the rotating motor 54 and rotating drive shaft 56,which in turn reduces wear on the pump motor 54 and other pumpcomponents.

Yet another advantage of the illustrative configuration is that noportion of the RCP 50 passes through the integral pressurizer volume.This simplifies design of the integral pressurizer 22 and shortens thelength of the drive shaft 56. However, since conventionally thepressurizer is located at the top of the pressure vessel, achieving thisarrangement in combination with vertically oriented pump motors 54 andvertically oriented drive shafts 56 entails reconfiguring thepressurizer. In the embodiment of FIGS. 1-5, the cross-section of thevessel head of the cylindrical pressure vessel 10 includes a narrowedportion defining a recess 76 of the integral pressurizer 22. The recess76 allows the pump motors 54 to be disposed at least partially in therecess 76 so as to provide sufficient room for the vertically mountedpump motors 54. The recess 76 should be large enough to accommodate thepump motor 54 during installation.

Still yet another advantage of placing the RCPs 50 at the head of thepressure vessel is that this arrangement does not occupy space lowerdown in the pressure vessel, thus leaving that space available foraccommodating internal CRDM units, a larger steam generator, or soforth.

The embodiment of FIG. 1-5 is illustrative, and it is contemplated thatthe various components such as the integral pressurizer 22, the RCPs 50,and so forth may be modified in various ways. With reference to FIGS.6-9, some additional illustrative embodiments are set forth.

With reference to FIGS. 6 and 7, an alternative embodiment of the uppervessel section is shown. The alternative embodiment of FIGS. 6 and 7differs from the embodiment of FIGS. 1-5 in that it has a modifiedpressure vessel 110 in which (1) the closure 64 for removing the vesselhead is omitted and (2) a differently shaped integral pressurizer 122 isprovided.

Unlike the integral pressurizer 22 of the embodiment of FIGS. 1-5 whichincluded the recess 76 for accommodating the RCPs 50, the integralpressurizer 122 is instead narrowed (that is, of smaller cross-sectiondiameter) over its entire height to accommodate the RCPs 50. Thisapproach has the advantage of providing more vertical space for mountingthe pump motors 54 (which can be especially advantageous if a long driveshaft is inserted when the motor is installed). A disadvantage of theintegral pressurizer 122 as compared with the pressurizer 22 is that theformer has reduced a reduced pressurizer volume due to its beingnarrowed over its entire height rather than over only the recess 76.

The embodiment of FIGS. 6 and 7 can employ the same RCP mountingarrangement as is used in the embodiment of FIGS. 1-5, includingmounting the RCP 50 at an opening of the pressure vessel that is toosmall for the pump casing or diffuser 58 to pass through. Since thevessel head is not removable to allow insertion of the diffuser 58,another access pathway such as an illustrative manway 130 is suitablyprovided for inserting the diffuser 58 into the pressure vessel. Themanway 130 is typically already required to provide access forperforming steam generator tube plugging or other maintenanceoperations—accordingly, no additional vessel penetration is required toenable the use of the disclosed pumps installed at small vesselopenings.

With particular reference to FIG. 7, installation of the RCPs 50 isshown. FIG. 7 shows the vessel head before installation of the RCPs 50,and with one RCP 50 positioned above an opening 132 that is sized to betoo small to pass the diffuser 58 but is large enough to pass theimpeller 52. In illustrative FIG. 7 the corresponding diffuser 58 isalready mounted inside the pressure vessel 110 on a support plate 134below the pressurizer separator plate 20. The support plate 134 alsoserves as the pressure divider separating the suction and dischargesides of the diffuser 58. In FIG. 7 the manways 130 are shown as capped.(Note that in FIG. 7 only two diffusers 58 are shown, with one thatcorresponds to the one RCP 50; more typically, all diffusers 58 will beinstalled via the open manways 130 into their respective openings of thesupport plate 134 before capping the manways 130.) To install theillustrative RCP 50 shown in FIG. 7, the impeller 52 and drive shaft 56are inserted through the opening 132 so as to position the impeller 52at its designed position inside the diffuser 58, and the flange 70 isbolted onto the opening 132 to form a seal (typically assisted by agasket, o-ring, or other sealing element at the flange 70.

With reference to FIG. 8, another embodiment is shown. This embodimentdiffers from the embodiment of FIGS. 6-7 in that (i) the openings 132are replaced by larger openings 142 that are sufficiently large to passthe entire assembly inclusive of the diffuser 58, and (ii) in theembodiment of FIG. 8 the RCP 50 is installed as a unitary pump assemblyincluding the motor 54, impeller 52, drive shaft 56, and diffuser 58. Inthis embodiment the unitary pump assembly includes the motor 54,impeller 52, drive shaft 56, and diffuser 58 secured together as saidunitary pump assembly 52, 54, 56, 58 with the impeller 52 disposed inthe diffuser 58 in the unitary pump assembly. FIG. 8 diagrammaticallyindicates the installation by showing the unitary pump assembly 52, 54,56, 58 (note, the impeller 52 is inside the diffuser 58 and hence notvisible in FIG. 8) arranged above a corresponding one of the openings142 preparatory to inserting the diffuser 58 into the opening 142. Thisembodiment has the advantage of enabling use of commercially availablepumps that may include the pump casing, and also enablesremoval/replacement of the diffuser 58 via the opening 142.Additionally, this embodiment facilitates pre-alignment of the impeller52 inside the diffuser 58 prior to installing the RCP 50 at the reactor.

The embodiment of FIG. 8 includes the manways 130. In the embodiment ofFIG. 8 the manways 130 are not used for installation/replacement ofdiffusers 58, but are typically used for performing steam generator tubeplugging or other maintenance operations (possibly including inspectionof the diffusers 58).

The illustrative embodiments are examples of contemplated variations andvariant embodiments; additional variations and variant embodiments thatare not illustrated are also contemplated. For example, while theillustrative PWR is an integral PWR including the internal steamgenerator 36, in some contemplated alternative embodiments an externalsteam generator is instead employed, in which case the feedwater inlet40 and steam outlet 42 are replaced by a primary coolant outlet port tothe steam generator and a primary coolant inlet port returning primarycoolant from the steam generator (alternative embodiment not shown).Moreover, while advantages are identified herein to not mechanicallycoupling the RCPs 50 to the internal steam generator, it isalternatively contemplated to couple the RCPs to the steam generatorinlet, for example by replacing the outlet plenum 62 and illustrativediffusers 58 with pump casings having outlets directly connected withprimary coolant inlets of the steam generator.

The preferred embodiments have been illustrated and described.Obviously, modifications and alterations will occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

I claim:
 1. A pressurized water reactor (PWR) comprising: a cylindricalpressure vessel with its cylinder axis oriented vertically; a nuclearreactor core disposed in the cylindrical pressure vessel; a separatorplate disposed in the cylindrical pressure vessel that separates thepressure vessel to define an integral pressurizer containing apressurizer volume disposed above the separator plate and a reactorvessel portion defining a reactor volume disposed below the separatorplate and containing the nuclear reactor core, wherein the separatorplate restricts but does not completely cut off fluid communicationbetween the pressurizer volume and the reactor volume; a hollowcylindrical central riser disposed concentrically with and inside thecylindrical pressure vessel in the reactor volume wherein a downcomerannulus is defined between the hollow cylindrical central riser and thecylindrical pressure vessel; a reactor coolant pump including animpeller disposed inside the pressure vessel in the reactor volume, apump motor disposed outside of the pressure vessel, and a drive shaftoperatively connecting the pump motor with the impeller, wherein theimpeller, pump motor, and drive shaft are secured together as a unitarypump assembly; a pump diffuser containing the impeller; an annularsupport plate disposed below the separator plate and having a centralopening coinciding with the hollow cylindrical central riser, the pumpdiffuser mounted on the annular support plate and not secured with theunitary pump assembly, the annular support plate defining a pressuredivider separating suction and discharge sides of the pump diffuser;wherein the drive shaft passes through an opening in the pressure vesselthat is at least large enough to pass the impeller; and wherein theimpeller of the reactor coolant pump is configured to impel coolantwater downward into the downcomer annulus.
 2. The PWR of claim 1,wherein no portion of the reactor coolant pump is disposed in thepressurizer volume, and the cross-section of the cylindrical pressurevessel includes a narrowed portion defining a recess of the integralpressurizer wherein the pump motor is disposed at least partially in therecess.
 3. The PWR of claim 1, wherein no portion of the reactor coolantpump is disposed in the pressurizer volume, and the pump motor at leastpartially overlaps a cross-sectional area of the reactor vessel portionof the cylindrical pressure vessel.
 4. The PWR of claim 1, wherein noportion of the reactor coolant pump is disposed in the pressurizervolume, and the drive shaft of the reactor coolant pump is orientedparallel with the cylinder axis of the cylindrical pressure vessel. 5.The PWR of claim 4, wherein the reactor coolant pump comprises N reactorcoolant pumps spaced apart at 360°/N intervals around the cylinder axisof the cylindrical pressure vessel where N is an integer greater than orequal to
 2. 6. The PWR of claim 1 further comprising: an internal steamgenerator disposed in the downcomer annulus.
 7. The PWR of claim 6,wherein into an outlet plenum that spaces apart the reactor coolant pumpand the internal steam generator, and the impeller of the reactorcoolant pump is arranged to discharge coolant water into the outletplenum.
 8. The PWR of claim 1, wherein the opening in the pressurevessel through which the drive shaft passes is too small for the pumpdiffuser to pass through.
 9. The PWR of claim 8, further comprising amanway disposed at or below the annular support plate that is largeenough for the pump diffuser to pass through.
 10. A pressurized waterreactor (PWR) comprising: a reactor coolant pump comprising a pumpmotor, an impeller, a driveshaft that operatively connects the pumpmotor and the impeller, and a pump diffuser, wherein the pump motor, theimpeller, and the driveshaft are secured together as a unitary pumpassembly with the impeller disposed in the pump diffuser and the pumpdiffuser is not secured with the unitary pump assembly; and acylindrical pressure vessel with its cylinder axis oriented vertically;a nuclear reactor core disposed in the cylindrical pressure vessel; aseparator plate disposed in the cylindrical pressure vessel thatseparates the pressure vessel to define an integral pressurizer volumedisposed above the separator plate and a reactor vessel portioncontaining the nuclear reactor core disposed below the separator plate;a support plate disposed below the separator plate, the pump diffusermounted on the support plate, the support plate defining a pressuredivider separating suction and discharge sides of the pump diffuser;wherein the unitary pump assembly is mounted on the cylindrical pressurevessel of the PWR with at least a portion of the pump motor disposedabove the separator plate and with no portion of the reactor coolantpump passing through the integral pressurizer volume.
 11. The PWR ofclaim 10, wherein the reactor coolant pump is mounted on the cylindricalpressure vessel with the drive shaft oriented vertically.
 12. The PWR ofclaim 10, wherein the reactor coolant pump is mounted on the cylindricalpressure vessel with the drive shaft oriented vertically and the pumpmotor disposed above the impeller.
 13. The PWR of claim 10, wherein thereactor coolant pump is mounted on the cylindrical pressure vessel withthe impeller and pump diffuser disposed in the reactor vessel portionbelow the separator plate.
 14. A pressurized water reactor (PWR)comprising: a pressure vessel; a nuclear reactor core disposed in thepressure vessel; a separator plate disposed in the pressure vessel thatseparates the pressure vessel to define an integral pressurizercontaining a pressurizer volume disposed above the separator plate and areactor vessel portion defining a reactor volume disposed below theseparator plate and containing the nuclear reactor core, wherein theseparator plate restricts but does not completely cut off fluidcommunication between the pressurizer volume and the reactor volume; ahollow central riser disposed inside the pressure vessel in the reactorvolume wherein a downcomer annulus is defined between the hollow centralriser and the pressure vessel; a reactor coolant pump including animpeller disposed inside the pressure vessel in the reactor volume, apump motor disposed outside of the pressure vessel, and a drive shaftoperatively connecting the pump motor with the impeller, wherein theimpeller, pump motor, and drive shaft are secured together as a unitarypump assembly; a pump diffuser containing the impeller, the pumpdiffuser not secured with the unitary pump assembly; an annular supportplate disposed below the separator plate and having a central openingcoinciding with the hollow central riser, the pump diffuser mounted onthe annular support plate, the annular support plate defining a pressuredivider separating a suction side above the annular support plate and adischarge side below the annular support plate; wherein the drive shaftpasses through an opening in the pressure vessel that is at least largeenough to pass the impeller; and wherein the impeller of the reactorcoolant pump is configured to impel coolant water downward from thesuction side above the annular support plate to the discharge side belowthe annular support plate.
 15. The PWR of claim 14 wherein the openingin the pressure vessel through which the drive shaft passes is too smallfor the pump diffuser to pass through.